The biogenesis of mitochondria depends on the cooperation of the nuclear and mitochondrial genome. Animal mitochondrial DNA (mtDNA) is a circular molecule of about 10 to the 7th power daltons, and codes for mitochondrial rRNA and tRNA, and some mRNA-like molecules. Earlier studies on mtDNA and mtRNA in Xenopus will be followed up by mapping the rRNA and tRNA regions on the DNA relative to the replication origin (D- loop). A similar study will be done with Drosophila mtDNA. Mitochondrial mRNA-like molecules will be isolated from Xenopus ovaries, and the regions in mtDNA coding for these RNAs will be mapped. Xenopus is chosen for this work because such studies will allow further work on the regulation of mitochondrial biogenesis in the development of oocytes and embryos. These studies on sequence arrangement, and a comparison of primary sequences of mtDNAs from different animals will yield information on the evolution of mtDNA. Our recent work on the fate of parental mtDNA in somatic hybrid cells will be continued. We have shown that mtDNA sequences from two different species may replicate within one cell and that stretches of different mtDNAs become linked to each other in a process akin to recombination. The detailed molecular arrangements of segments in recombinant mtDNA will be studied to obtain information on recombination mechanisms in animal cell mitochondria. This subject is of special interest since it has been suggested that circular dimers and other oligomers of mtDNA may arise by recombination. Such dimers are found only in tumor cells and their presence constitutes one of the strong biochemical correlates of the tumor state. Hybrid cells, especially those carrying mitochondrial mutants, will be used in studying nucleo- cytoplasmic interactions and the regulation of the expression of mtDNA.